Globular cluster Messier 92 (M92, NGC 6341) is one of the more conspicuous
globular clusters. Situated in constellation Hercules, it is nevertheless
second only within that constellation, after bright
M13.

M92 is one of the original discoveries of
Johann Elert Bode, who found it on
December 27, 1777.
Charles Messier independently
rediscovered it and cataloged it
on March 18, 1781, the same day as he cataloged another 8 objects, all of them
Virgo Cluster galaxies
(M84-M91).
It was William Herschel who first
resolved it into stars in 1783.

According to newer sources, M92 is about 26,000 light years distant, only
little more than its brighter apparent neighbor M13.
From its HRD (or CMD), it may be a bit younger than M13 as its turnoff point
is shifted to the brighter and bluer end.
A semi-recent estimate of M92's age has given a value of about 16 billion
years (anyway more than 14 billion years), see e.g. the diagram in
Sky & Telescope, January 1996, p. 22 (text on p. 20).
However, this value is now again under discussion because of the general
modifications of the distance scale of the universe, implied by results of
ESA's astrometrical satellite Hipparcos: These results suggest that M92, as
well as most other globular clusters, may be at a 10 per cent larger distance;
therefore, the intrinsical brightness of all their stars must be about 20 %
higher. Considering the various relations which are important for understanding
stellar structure and evolution, they should also be roughly 15 % younger,
in a preliminary off-hand estimate (or about 12-14 billion years).
For M92, W.E.
Harris' globular cluster database lists an only little modified value of
26,700 (former 26,100) light years, though, so that the quoted result might
stay valid after all.

M92 is a splendid object, visible to the naked eye under very good
conditions and a showpiece for every optics. It is only slightly less bright
but about 1/3 less extended than M13: its 14.0' angular extension
corresponds to a true diameter of 109 light years, and may have a mass of
up to 330,000 suns.

Only about 16 variables have been discovered in this globular, 14 of which are
of RR Lyrae type, while one of them is one of the very few eclipsing binaries
in globular clusters, of W Ursae Majoris type.
Although Burnham claims it is not well understood why eclipsing binaries
are so rare in globulars, it appears to the present author that there may be
a simple answer: In these dense stellar agglomerates, close encounters occur
frequently, so that binary systems will be disturbed, and on the long term,
will be destroyed.

M92 is approaching us at 112 km/sec.

Interesting trivia on M92: As the ecliptical coordinates for M92, Longitude =
249.9 deg, Latitude = 65.9 deg, indicate, the Earth's North Celestial Pole
occasionally passes closer than 1 degree of this cluster, at periods of the
precession of Earth's axis (about 25,800 years). So this cluster becomes a
"Polarissima Borealis", or "North Cluster", in about 14,000 years (16,000 AD),
as it was about 12,000 years ago (10,000 BC).